The present invention relates to an engine valve driving apparatus, and more particularly to an engine valve driving apparatus by which an engine valve is controlled to work according to engine operating conditions.
In the field of internal combustion engines, engine valve driving apparatus are generally well known by which an engine valve is controlled to work in cooperation with a valve driving cam according to engine operating conditions. One such engine valve driving apparatus as described in Japanese Patent Unexamined Publication No. 59-155,515 entitled "Valve Closing Device", laid open Aug. 4, 1984, relates to a device comprised of a camshaft, a valve driving cam rotatably supported by the camshaft, and a plunger disposed in the camshaft which causes the interruption of relative rotation between the camshaft and the valve driving cam which is controlled to move radially by a hydraulic system according to engine operating conditions.
In the device taught by the above-mentioned Japanese Patent Unexamined Publication, the plunger is usually forced by a biasing spring to partially radially protrude from the camshaft and to enter an engaging bore formed in the valve driving cam when no oil pressure is exerted on the plunger, interconnecting the camshaft and the valve driving cam. Due to this rigid interconnection, the valve driving cam and camshaft can rotate as an integral whole, so as to drive suction and exhaust valves to open and close in a predetermined timing. If oil pressure is exerted on the plunger, then the plunger retracts into the camshaft against the biasing spring to dissolve or interrupt the interconnection between the camshaft and the valve driving cam, permitting the valve driving cam to rotate about and relative to the camshaft. That is, the camshaft runs idle with the valve driving cam rendered inoperative.
There is another type of engine valve driving apparatus which is taught by Japanese Patent Unexamined Publication No. 58-91,317 entitled "Variable Timing Mechanism", laid open May 31, 1983. This mechanism has at least two valve driving cams which are selectively brought into operational engagement with an engine valve according to engine operating conditions; namely, high speed and low speed engine operating conditions. These valve driving cams are fixedly mounted side-by-side on a hollow cylindrical member with an annular hub provided at one end thereof. The hollow cylindrical member is supported in sliding fit by the camshaft for axial movement. For causing the axial movement of the hollow cylindrical member, there is an axially extending shifting shaft with a fork fixed thereto and engaged by the annular hub of the hollow cylindrical member. By moving the shifting shaft axially, the two cams are interchangeably brought into cooperation with the engine valve to drive it in different timings according to engine operating conditions. such an engine valve driving apparatus as described above is widely applicable to engines for the purpose of effecting various engine controls; for instance, changing timings in which the engine valve opens and closes according to engine operating conditions, stopping valve motion under a specific engine operating condition, or the like.
However, the device taught by the above-mentioned Japanese Patent Unexamined Publication No. 59-155,515 is structurally unfavorable in view of strength because of the fact that what is subjected to a reaction caused as the result of an action of he engine valve is only the plunger when the valve driving cam rotates integrally with the camshaft in one body. In this respect, the device taught by the above-mentioned Japanese Patent Unexamined Publication No. 58-91,317 is more or less advantageous on comparison with the former one not only because the valve driving cam is supported over the whole width thereof but also because a plurality of axially extending splines by which the valve driving cam and camshaft are coupled to each other support reaction froce from the engine valve, individually. Although having such structural advantages, nevertheless, the provision of the shifting shaft and fork unavoidably makes the device not only large but also complex.